Coated abrasive sheet materials, including one type known in the trade as waterproof sandpaper, and another type known as dry finishing paper, have been used for metal and wood finishing for many years. The substrate used for the backing of the waterproof sandpaper is customarily a very porous, flexible paper impregnated with from about 15% to 40% of a synthetic resin or elastomer to provide toughness and durability, especially when wetted. Current production techniques require that this very porous paper be coated with a synthetic resin barrier coating to seal the surface before applying the adhesive or "make coat" which bonds the abrasive grains or grit to the substrate.
The substrate employed for dry finishing paper is much stiffer and non-porous and in such products the bonding adhesive coat is usually applied directly to the paper without first applying a barrier coating.
Adhesives used for bonding the grit to waterproof sandpaper include epoxy resins, epoxy esters, phenolic resins, and alkyd resins in suitable solvents. Bonding adhesives for dry finishing paper on the other hand are usually animal glues and water based synthetic resins.
In the conventional production of both kinds of paper, the bonding adhesive is dissolved or dispersed in a solvent or carrier and the mixture is then applied by a pressure coating nip to the continuous web substrate. The abrasive grit particles are then deposited on the moving web before the solvent or carrier is driven off, and while the adhesive is still fluid. The grit particles are usually oriented or aligned by electrostatic means to maximize abrasive or cutting properties. No external pressure is applied to the particles after deposition, as this tends to destroy the alignment of particles, while burying the particles in the backing, both of which are undesirable. After the solvent or carrier is driven off, the web carrying the adhesive and grit is passed through an oven which heats the material for times ranging from several minutes to several hours to cure the thermosetting resins usually employed as adhesives and to firmly bond the grit therein. Dry finishing paper adhesives usually do not require heat curing, although heat may be employed to drive off the fluid carrier.
After the grit is firmly bound to the backing, a "grain size" coating is applied over the layer of abrasive particles to complete the fabrication step. The grain size coating is usually a hard, thermosetting resin or animal glue which anchors the particles more firmly so that they remain aligned for maximum cutting ability.
It should be appreciated that as indicated above the making of abrasive paper is a slow, continuous process. Because the adhesive coating, particle deposition, drying and curing are carried out in a continuous process, many variables such as the solution viscosity, coating weight, grit deposition weight, web tension, drying rate and curing temperature must be controlled simultaneously. If not properly controlled, problems often arise such as curling and breaking of the moving web, excessive penetration of adhesive into the web, or too little bonding strength. Also, if the backing is pre-coated with a barrier coat prior to applying the grit bonding adhesive, poor adhesion between the barrier coating and adhesive coating is often encountered unless the two coatings are carefully formulated and tested. It is virtually impossible to supply an abrasive backing with a single barrier coating which is compatible with all the many types and variations of adhesives currently being employed by abrasive paper manufacturers. Thus barrier coatings are individually tailored to meet the individual requirements of the many abrasive manufacturing facilities in the world.
The recent emphasis on control of air pollution has complicated the situation further. Many abrasive manufacturers who in the past relied on solvent-based resins for making wet or dry abrasive paper are now being forced to use water-based systems to reduce such air pollution. This change necessitates extensive testing of new adhesive systems and the development of new barrier coatings. Although it may be possible to solve many of these problems with current techniques, the alternative proposed here, a backing with a heat-activatable adhesive pre-applied, provides an economical and efficient system whereby abrasive manufacturers can eliminate or minimize many of these problems.
Since in this preferred system, there is no need for the abrasive paper manufacturer to apply a separate adhesive prior to depositing the grit, the abrasive paper manufacturing operation is simplified considerably. In this system a simple means for heating the precoated backing material replaces the more complicated adhesive coating equipment. Problems such as curling and wrinkling of the moving web are thereby minimized. Air pollution which accompanies the use of solvent-based adhesives is also eliminated along with current problems with respect to poor adhesion between the adhesive coating and barrier coating, since these two coatings are replaced by a single heat-activatable adhesive coating preapplied by the backing paper supplier. While the cost of a raw materials used in preparing the backing material of this invention are increased slightly over conventional backing material, because the preferred water-based, heat-activatable adhesives applicable to this invention are more expensive than the barrier coatings currently used, and because the coating weights are in most instances greater than the barrier coating weights of current products, the other advantages discussed above, including the use of a single coat rather than double coat system contribute to make this invention attractive to abrasive paper manufacturers.